Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

decreasing source temperature, about 12.5 percent for each 6°C (10°F ) drop

in the source temperature. For example, the capacity goes down to 50 per-

cent when the supply water temperature drops to 93°C (200°F ). In that case,

one needs to double the size (and thus the cost) of the chiller to achieve the

same cooling. The COP of the chiller is affected less by the decline of the

source temperature. The COP drops by 2.5 percent for each 6°C (10°F )

drop in the source temperature. The nominal COP of single-stage absorption

chillers at 116°C (240°F ) is 0.65 to 0.70. Therefore, for each ton of refriger-

ation, a heat input of (12,000 Btu/h)/0.65 
18,460 Btu/h is required.

At 88°C (190°F ), the COP drops by 12.5 percent and thus the heat input

increases by 12.5 percent for the same cooling effect. Therefore, the eco-

nomic aspects must be evaluated carefully before any absorption refrigera-

tion system is considered, especially when the source temperature is below

93°C (200°F ).

Another absorption refrigeration system that is quite popular with

campers is a propane-fired system invented by two Swedish undergraduate

students. In this system, the pump is replaced by a third fluid (hydrogen),

which makes it a truly portable unit.

634 | Thermodynamics

All the refrigeration systems discussed above involve many moving parts and

bulky, complex components. Then this question comes to mind: Is it really

necessary for a refrigeration system to be so complex? Can we not achieve

the same effect in a more direct way? The answer to this question is yes. It is

possible to use electric energy more directly to produce cooling without

involving any refrigerants and moving parts. Below we discuss one such sys-

tem, called thermoelectric refrigerator.

Consider two wires made from different metals joined at both ends (junc-

tions), forming a closed circuit. Ordinarily, nothing will happen. However,

when one of the ends is heated, something interesting happens: A current

flows continuously in the circuit, as shown in Fig. 11–23. This is called the

Seebeck effect,in honor of Thomas Seebeck, who made this discovery in

1821. The circuit that incorporates both thermal and electrical effects is called
      a thermoelectric circuit,and a device that operates on this circuit is called a
      thermoelectric device.
      The Seebeck effect has two major applications: temperature measurement
      and power generation. When the thermoelectric circuit is broken, as shown in
      Fig. 11–24, the current ceases to flow, and we can measure the driving force
      (the electromotive force) or the voltage generated in the circuit by a voltmeter.
      The voltage generated is a function of the temperature difference and the
      materials of the two wires used. Therefore, temperature can be measured by
      simply measuring voltages. The two wires used to measure the temperature in

TOPIC OF SPECIAL INTEREST* Thermoelectric Power Generation and Refrigeration Systems

*This section can be skipped without a loss in continuity.

Metal A

Metal B

I

I

FIGURE 11–23

When one of the junctions of two
dissimilar metals is heated, a current I
flows through the closed circuit.

+–

Metal A

Metal B

I = 0

V

FIGURE 11–24

When a thermoelectric circuit is
broken, a potential difference is
generated.